A capacitive sensor is one type of sensor that is employed for measuring relative humidity (RH) concentrations in ambient air. FIG. 1 illustrates a cross-section of a conventional capacitive sensor 100 that is used for measuring RH. As shown in FIG. 1, spaced sensor electrodes 102, 104 and 106 are formed on a substrate 101 as the “fingers” of an interdigitated capacitive structure, although conventional capacitive sensors have also employed parallel-plate capacitor configurations. It will be recognized that the capacitive structure may be formed by many electrodes arranged as shown in FIG. 1. Capacitance of sensor 100 increases with RH due to absorption of high polarity water molecules. Generally the water molecules can be absorbed and desorbed. Thus, capacitance measurements obtained between the electrodes may be utilized to determine increasing and decreasing relative humidity levels. Sensor electrodes may be any of a wide variety of conductive materials. Substrate 101 may be any of a wide variety of substrates and may be in one non-limiting example a semiconductor substrate that includes a wide variety of integrated circuit layers (not shown) as is known in the art. For example, U.S. Pat. No. 8,007,167 to Cummins, the disclosure of which is expressly incorporated herein by reference, provides a capacitive sensor formed on an integrated circuit substrate.
The sensor electrodes may be covered by a passivation layer 103 and further overlaid with a sensing layer 105 of polyimide. Alternatively, sensing layer 105 may be utilized without the inclusion of a passivation layer 103. In operation, the sensing layer 105 is exposed to the ambient conditions under which a measurement is desired. Thus, at least a portion of the upper surface of the sensing layer 105 may be an air/dielectric layer interface and layer 105 may be considered an ambient condition humidity-sensitive layer. Capacitive RH sensors 100 such as illustrated in FIG. 1 are typically provided in a package with an opening defined in the package that is provided for allowing ingress of ambient air or other gas into the package.
Still referring to FIG. 1, concentration of water vapor in the ambient air impacts (and changes) the dielectric constant of the sensing layer 105 as differing concentrations of water vapor in the ambient air will impact the amount of ingress of water molecules into the sensor dielectric material of sensing layer 105. Thus, by measuring the capacitance between the electrodes the RH concentrations in the ambient air may be inferred. As shown in FIG. 1, the electric fields between the electrodes may include fields 110a contained in the passivation layer 103, fields 110b which pass in part through the sensing layer 105, and other parasitic fields (not shown). In operation, changes in capacitance of sensor 100 due to changes in the dielectric constant of the sensing layer 105 are utilized to detect the ambient RH conditions. However, various components of the capacitive measurement may be impacted by several factors, i.e., such as temperature changes, chemical contaminants, physical contaminants, etc. These factors thus impact the accuracy of the detection of the ambient RH conditions using sensor 100.
Polyimide-based capacitive RH sensors also suffer from stability problems as they age at conditions of elevated humidity and temperature. This results in decreasing RH sensor accuracy upon subjection of a polyimide capacitive RH sensor to high temperature and humidity environments over time. With exposure to elevated humidity and temperature levels the measured capacitance (sensor response) obtained from a polyimide-based capacitive RH sensor slowly drifts with time above the reference capacitance value that corresponds to actual environmental RH conditions, and that should be exhibited by the sensor in the absence of the effects of temperature and humidity. This causes the aged sensor to indicate a false increase in RH that is referred to as “drift”. A related term is “shift”, which is the similar difference in sensor response reading at non-elevated ambient humidity and temperature conditions following a high RH exposure. It has been proposed to select or alter the chemical properties of a polyimide for increased stability at high temperature and humidity in an attempt to improve accuracy of capacitive RH sensors after exposure to elevated temperature and humidity.